Far-left presidential candidate Jean-Luc Melenchon at a political rally in Toulouse last weekend.
Photograph: Regis Duvignau/Reuters

With two convinced Eurosceptics and an equally fervent pro-European among the four contenders with a chance of reaching the run-off, France’s too-tight-to-call presidential election could conceivably break – or make – the EU.

European officials and diplomats appear generally unconvinced that France, a core member of the bloc, will actually leave – an idea touted, not always forcefully, by the hard-left candidate, Jean-Luc Mélenchon, and his far-right rival, Marine Le Pen.

More worrying, and perhaps more likely, is the prospect of Paris turning its back. The EU would not survive Frexit, though “that seems quite remote”, said one senior diplomat. “But active inside opposition would almost be worse.”

Battered by Brexit and facing a range of other problems including an unresolved migrant crisis, slow economic growth and a clutch of increasingly rebellious capitals, the EU could badly do with some good news.

A victory for Emmanuel Macron, the liberal, overtly pro-EU centrist who, polls predict, will win if he is one of the two second round finalists, would offer France a chance to reform, and Europe a chance to rebound.

A Macron win could also suggest that, after Britain’s decision to leave and the shock election of Donald Trump in the US, “the demise of liberalism, internationalism and the EU ... may not be inevitable,” Grant added.

It might also – if Macron can implement his planned labour and other structural reforms – revive the stuttering Franco-German motor that has traditionally powered the EU but been hobbled in recent years by France’s ailing economy.

A President Macron would, certainly, make the UK’s Brexit negotiations tougher. He has already warned there can be no “caveat or waiver” to the bloc’s “unbreakable” position that its first priority must be to defend its own interests.

With the eurozone economy starting to recover and after the defeat of the anti-EU Geert Wilders in the Netherlands, Macron – dubbed “Brussels’ sweetheart” by his opponents – would be “a huge boost”, one European commission staffer said.

Unfortunately for the EU, however, the former banker and Socialist economy minister, who has never held elected office, may be favourite to win the second round – but he is not sure to make it past the first.

The four frontrunners – including the scandal-hit rightwing candidate, François Fillon, who dislikes federalism and wants a more intergovernmental EU, but has no plans to change France’s relationship with the bloc – are so close it is impossible to say which two will reach the run-off.

The prospect of Le Pen, the far-right, anti-EU Front National leader, or the radical left Mélenchon winning has set alarm bells ringing Brussels. While their views on immigration are diametrically opposed (Le Pen wants to pull France out of the Schengen border-free zone), there is little between the two candidates’ positions on the euro and EU membership.

Mélenchon’s so-called “Plan A” is a collective renegotiation, by those who want it, of the EU’s treaties and agreement on fundamental reform, including major changes to the euro. His Plan B, should Plan A fail, is a unilateral French exit – although he has recently sought to calm fears that this is his objective.

The far-left veteran says the outcome of talks would in any event be put to a referendum – something Le Pen has also promised, after a “renegotiation of France’s membership” aimed at “full restoration of national sovereignty” including a return to the franc.

On the face of it, her aims are not compatible with France’s membership of the EU as it currently exists.

If the EU27 appear relatively sanguine about the chances of Frexit and a French departure from the euro – neither would be possible without the other – it is partly because a French president without a parliamentary majority is hamstrung.

This could hit Macron, who – if he won – would need, after legislative elections in June, to cobble together a complex coalition of the willing made up of centrists from left and right and newly elected members of his En Marche! movement.

It would be infinitely more incapacitating for Le Pen or Mélenchon, both of whom are highly unlikely to win parliamentary backing for a referendum on France’s EU membership, which is anchored in the country’s constitution.

Should the new president decide to bypass parliament and France’s constitutional court and go direct to the people, surveys suggest few French voters would willingly embrace the chaos of a euro exit economists estimate could cost €180bn (£150bn).

But even without a full-blown Frexit, a populist, Eurosceptic French president could pose huge problems to the bloc – beginning with Brexit negotiations, on which the EU27 have so far shown a genuinely astonishing degree of unity.

But Le Pen has hailed Britain’s vote to leave as “the first real blow to the old order” and “the birth of a new Europe”, and could certainly be expected to push hard for terms that would do anything but discourage others from leaving.

Mélenchon has also called Brexit “the beginning of the end of an era” and “an earthquake for the EU as we know it”. His campaign programme calls explicitly for Britain’s exit process to be “without vengeance or punishment”.

But the potential for longer-term confrontation goes much further than EU disunity over Brexit terms. Both candidates’ economic programmes, for example, include major public spending hikes that would break EU budget deficit rules.

Le Pen’s plans to tax imports and low-paid labour from eastern Europe would be incompatible with the single market; Mélenchon’s promises to veto free-trade pacts and end European Central Bank independence would be anathema in Brussels.

In the end, it could be that the mere fact of a President Le Pen or Mélenchon, flouting rules, contesting proposals and withholding overall support, would be enough to hobble the EU – when it most needs to advance. For Brussels, few European elections have felt so significant.